Silver halide emulsions of surface latent image type are widely employed in photographic light-sensitive materials and photographic papers.
The surface latent image type silver halide emulsions are usually sensitized by chemical sensitization with sulfur sensitizers, noble metal sensitizers, and the like. However, chemical sensitization for imparting higher sensitivity to surface latent image type silver halide emulsions often raises problems, such as that increased fog results on development, or that fog increases with the passage of time before use after the emulsion is coated on a support. Such tendency is particularly conspicuous when the grain size of the silver halide crystals is increased for obtaining a silver halide emulsion having enhanced sensitivity.
In order to overcome these disadvantages, various additives have hitherto been proposed. For example, a method is known for stabilizing a silver halide emulsion by restraining fog through use of potassium ferricyanide in the precipitation step or physical ripening step, as disclosed in Japanese Patent Application (OPI) No. 1835/72 (the term "OPI" as herein used means "unexamined published application").
Further, Japanese Patent Publication Nos. 41056/76 and 56855/83 disclose a method for inhibiting fog and thus stabilizing a silver halide emulsion by using an organic halogen compound in the chemical ripening step.
Furthermore, East German Pat. No. 7376 describes a method of inhibiting fog by adding a thiosulfonate, exemplified by sodium benzenethiosulfonate, to an emulsion during chemical ripening or by the time immediately before coating.
Although use of the above-described additives, such as potassium ferricyanide, organic halogen compounds, and thiosulfonates has succeeded in inhibiting fog or increases in fog over time, the latent image that is formed by exposure of a photographic light-sensitive material to light is unavoidably oxidized when left to stand for a long time until development, e.g., for a period of from several days to several months, and, as a result, development of such as exposed material forms an inferior image. This phenomenon is generally called fading of the latent image.
In addition, use of potassium ferricyanide as conventionally proposed gives rise to an environmental pollution problem.
Accordingly, development of a surface latent image type silver halide emulsion free from the above-described disadvantages has been strongly desired.